As a result of the constantly increasing integration density in the semiconductor industry, photolithographic masks have to project smaller and smaller structures. In order to fulfil this demand, the exposure wavelength of photolithographic masks has been shifted from the near ultraviolet across the mean ultraviolet into the far ultraviolet region of the electromagnetic spectrum. Presently, a wavelength of 193 nm is typically used for the exposure of the photoresist on wafers. As a consequence, the manufacturing of photolithographic masks with increasing resolution is becoming more and more complex, and thus more and more expensive as well. In order to use significantly smaller wavelengths, lithography systems for the extreme ultraviolet (EUV) wavelength range (approximately 10-16 nm) are presently in development.
Photolithographic masks have to fulfill highest demands with respect to transmission, planarity, pureness and temperature stability. In particular, the surface of reflective masks for EUV radiation coated with the reflective structure has to be plane within the range of about 1 nm in order to avoid aberrations of the desired structure in the photoresist of the wafer. These challenges also apply for other EUV reflective optical elements, as for example mirrors used in the beam path of EUV lithography systems.
The above mentioned challenges require highly precise techniques for the production of the substrates of EUV optical elements. However, even the best production techniques cannot guarantee surface variations below 1 nm. Moreover, the fabrication of mask blanks and/or EUV optical elements from mask blanks may additionally induce further defects in the EUV substrates, and/or thus also in the EUV optical elements. It is therefore necessary to correct defects of EUV optical elements in order to establish an economical production process for these components.
On the other hand, an extremely careful and precise handling and holding of EUV mask blanks and/or EUV optical elements is necessary in order to avoid as far as possible mechanical abrasion and/or the formation of particles from the EUV optical element which may deteriorate the function of an EUV lithography system. Since an EUV optical element is used to expose a large number of semiconductor substrates or wafers, a high effort in terms of production and handling of EUV optical elements is almost always justified.
In order to fulfill these handling requirements, mask EUV blanks are held on an electrostatic chuck during the fabrication of an EUV optical element. Further, EUV masks are also held with an electrostatic chuck in the lithography system during the wafer illumination. As the substrate of EUV optical elements typically comprises a dielectric material or a semiconducting material, an electrically conducting layer has to be deposited on the rear side of a substrate in order to be able to hold the substrate with an electrostatic chuck during the fabrication and/or operation of the optical element. Typically, the electrical sheet resistance (Rs) of such electrically conductive layers has to be lower than several hundreds of Ω/□, preferably lower than 100Ω/□.
As already mentioned, errors already introduced in the substrate during the substrate production and/or introduced during the fabrication process of the EUV optical element have to be corrected at the end of the production process of the EUV optical element. Moreover, defects may evolve in the course of the operation of an EUV mask in a lithography system.
It is already known that a surface of an EUV optical element can be modified in a controlled manner in order correct planarity and/or placement defects by applying ultra-short laser pulses into the substrate of an optical element (U.S. Pat. No. 6,841,786 B2).
This defect compensation occurs through the rear side of the EUV optical element as the ultra-short laser pulses cannot penetrate the multi-layer structure, which forms the reflective optical element arranged on the front surface of the EUV optical element. Consequently, the electrically conducting layer deposited on the rear side for holding the EUV optical element with an electrostatic chuck has also to be optically transparent for the ultra-short laser pulses.
It is known that nitrides of metals, such as CrN, can be used as backside coating of lithography masks. They are typically used in their stoichiometric composition with a typical bulk resistivity (ρ) of 640 μΩ·cm. For example, a layer of 100 nm thick provides a surface resistance (Rs) of about 64Ω/□. At these thicknesses such a coating is not transparent. When the thickness is reduced to 25 nm or 5 nm, Rs becomes 256Ω/□ and 1280Ω/□, respectively, while the transparency (T) remains at about 15 and 55%, respectively (FIG. 1). Note that T is an average value for the visible range (400 nm-700 nm) and Rs is calculated from p by dividing by the layer thickness (t), i.e. Rs=ρ/t.
The trade-off between Rs and T for CrN is such that it cannot meet the typical electrical requirements of lithomask if the coating has to be transparent. Cr has a better trade off as its T is larger for films which have relatively low Rs (FIG. 1). For example, T is easily more than 20% for films with Rs of about 13Ω/□. However, Cr is not as strong as CrN from a mechanical resistance (hardness) point of view, for example against sliding, scratching and abrasive forces. In particular, where optical transmission was not a requirement, CrN is the premium material for back-side coating of lithography masks.
It is known that borides and carbides have similar properties to nitrides (see for example, Synthesis, properties and applications of nanoscale nitrides, borides and carbides by Liqiang Xu et Al. Nanoscale 2012, 4, 4900-4915) i.e. they can be electrically conductive, have high mechanical durability and become transparent when sufficiently thin (Transparent, Flexible and Conductive 2D Titanium Carbide (MXene) Films with High Volumetric Capacitance by Chuanfang Zhang et Al. Advanced Materials 2017, 1702678, DOI: 10.1002/adma.201702678). It is also known that Mo, W, Ti, Zr, Hf, V, Nb, Ta have similar chemical properties to Cr and can form similar nitrides, borides and carbides (Optical Properties of dense zirconium and tantalum diborides for solar thermal absorbers, by Elisa Sani et Al. Renewable Energy, vol. 91 (2016) 340-346).
Where optical transmission, electrical conductivity and mechanical resistance are all requirements, amongst different structures, patent application EP 11185280 proposes to use multi-layered back side coating made of Cr and CrN. Cr allows to achieve sufficiently low Rs and high T while the top CrN layer provides the strong mechanical resistance. However due to the intrinsic mechanical weakness of Cr and the difference in thermal and mechanical properties of the two materials (Cr and CrN), adhesion problems may occur. This is shown in FIG. 3 where it is clear that the Rs value after erasure testing change significantly (more than 6%) for Cr and multilayer Cr/CrN. Ultrathin metal containing structures can be also made with metals similar to Cr, e.g. EP11185280 lists several examples and also metals listed above, and hard compounds different from nitrides, e.g. borades and carbides. However these structures are likely to suffer from the same potential mechanical weakness of Cr/CrN and their applicability depends on the severity of the operating conditions.
As being held on an electrostatic chuck, rear side coatings of EUV optical elements have in addition of being electrically conducting and optically transparent also to fulfil specific mechanical requirements. For example, the pins of an electrostatic chuck or particles may indent in the surface coating on the substrate rear side. Moreover, the rear side coating has to withstand the lateral accelerations occurring during the mask scanning process. For this reason, as already explained in the US 2006/0115744 A1, the coating on the rear side of the substrate of an EUV optical element has to withstand abrasion during the handling of mask blank and/or the EUV optical element with an electrostatic chuck. Further, the electrical conductivity of the rear side coating has to be high enough, so that the mask blank and/or the EUV optical element can securely be handled with an electrostatic chuck. Moreover, the rear side coating has to be optically transparent, so that ultra-short laser pulses with a high optical intensity can be applied through the coating into the substrate of the mask blank and/or the EUV optical element.
By varying the atmosphere during deposition of the Cr atoms on a substrate one can obtain different CrNy compositions. More specifically the atmosphere during deposition can contain N2 and depending on the quantity of it, compositions with y varying from 0 to 1 or even more than 1 can be obtained. The most commonly used sputtering and evaporation techniques to deposit CrNy employs an atmosphere composed of N2 and Ar. Critical parameter to determine the resulting CrNy composition (i.e. the value of y) is the ratio between N2 and Ar, or more properly the ratio between the flux of N2 and total flux (sum of N2 and Ar), N2/(N2+Ar) ratio. The larger this ratio the larger the y value (Journal of Non-Crystalline Solids 218 (1997) 68-73).
It is therefore one object of the present invention to provide a coating and a method for depositing the coating on a substrate of a photolithographic mask that is electrically conductive, optically transparent and additionally has suitable mechanical properties and better adhesion.